Author Topic: Resistance of resistor leads ?  (Read 621 times)

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Offline lordvader88

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Resistance of resistor leads ?
« on: June 23, 2019, 10:16:45 am »
I use the trimmed leads of 1/8W and 1/4W resistors, as breadboard wire. So how much current could I really pass before it would matter over a few centimeters ?

I'm just wondering, since I just soldered on some big heavy wire wire, to handle 1A under 30V, just to go a few centimeters, I surely could just use resistor leads still, on the perf.board
 

Offline mariush

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Re: Resistance of resistor leads ?
« Reply #1 on: June 23, 2019, 10:32:52 am »
Diameter is probably ~0.45mm .... equivalent to ~ AWG 25.
A copper wire with AWG 25 gauge has ~ 106 mOhm per meter of wire, but resistors use tin plated steel or other materials with higher resistance... so let's go with 150 mOhm per meter or 1.5 mOhm per cm.

Let's go with 4cm of wire because it makes math easier ... that's 6 mOhm of resistance ... or 0.006 ohm

V = I x R , P = I^2xR

At 20A , you're dropping 20 x 0.006 =  0.12v so you shouldn't use such a lead to power something with 3.3v at up to 20A (for example a bunch of lithium batteries in parallel)

P = 20x20x0.006 = 2.4w dissipated in 4 cm of wire ... congrats you got yourself a small lightbulb
 

Offline MosherIV

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Re: Resistance of resistor leads ?
« Reply #2 on: June 23, 2019, 10:33:11 am »
I once asked a similar question once about current rating for wire.
The answer is........

Wire will pass any current you like BUT how hot it gets is dependant on the wire parameters.

To answer your question, yes at 0.5A upwards you are right to question whether a resistor lead can handle the current safely.
Compare the resistor lead to similar cable/wire ratings and you will have an approximate answer.

I think 1A is roughly the current limit/capability of perf/strip board.
You can increase it by soldering over the track or by adding parallel lead. Dave did a video about pcb track current capability.
 

Offline tooki

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Re: Resistance of resistor leads ?
« Reply #3 on: June 23, 2019, 10:40:16 am »
Current carrying capacity (ampacity) depends on the thickness of the leads*, which varies wildly. Cheap chinese 1/8W resistors have super thin leads (which might not even be copper), while a name-brand 2W resistor will have thick copper leads.

If you really want to know, first use a magnet to make sure they're not steel, and then use calipers to measure the diameter. You can then use any of the many online wire gauge tables (e.g. on wikipedia) to determine what gauge it is. (Or use your middle school geometry to calculate the cross section in mm2, if you prefer metric wire sizes.) Then once you've determined this, you can refer to an ampacity table to determine its current carrying ability.

But for 1A, you only need around 29ga wire, which is hair thin. Any name-brand component lead is going to be thick enough to carry several amps, but even the cheap Chinese ones should be thick enough for an amp or two. (The voltage is irrelevant to wire gauge, but defines how good the insulation must be. But 30V is nothing at all. It matters once you get to mains voltages and beyond. High voltage stuff, like in the kV, is where insulation really starts to become critical.)


* and on cooling. Bare wire hanging in free air can cool better than insulated wire in a bundle of cables, for example. The better the cooling, the more current a given wire can carry. A PCB can act as a heat sink to an extent, so for example if you solder a wire to a bunch of unused perfboard pads, they'll both increase heat sinking, as well as increasing the effective thickness of the wire, reducing resistance. But for 1A that's not even needed.
« Last Edit: June 23, 2019, 10:41:52 am by tooki »
 

Offline 3roomlab

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Re: Resistance of resistor leads ?
« Reply #4 on: June 23, 2019, 11:31:39 am »
I remembered reading this fusing resistor wire thing
5A ~ 6A ? from a 1/8 watter?
https://blog.smartere.dk/2018/09/18650-lithium-ion-battery-packs-1s80p/

but chinese resistors?
we are all playing with electrons that have NOT been washed for billions of years. Arent they suppose to smell bad? If you dont wash the electrons, your electricity is dirty. its suppose to be a joke, but some went and washed their hands.
 

Offline HighVoltage

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Re: Resistance of resistor leads ?
« Reply #5 on: June 23, 2019, 11:52:49 am »
It also largely depends if you send DC or pulsed current through the wire?
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline magic

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Re: Resistance of resistor leads ?
« Reply #6 on: June 23, 2019, 12:26:42 pm »
It's best to measure that kind of things when they aren't rated by the manufacturer.

I've built a tiny, TL431-based, USB powered 100mA current source for such measurements. Test current goes through the DUT and develops 0.1mV of voltage on it per each 1mΩ of resistance. Works on wires, cables, switches, connectors, anything.

The basic idea can be found in TI TL431 datasheet. I've modified it a bit to avoid error from base current, another possibility is to simply replace the BJT with a MOSFET.
 

Offline ArthurDent

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Re: Resistance of resistor leads ?
« Reply #7 on: June 23, 2019, 12:34:51 pm »
Perf board or breadboard, why worry just about the leads and ignore what could be a bigger problem.

Quote
"Breadboard is commonly rated for five volts at one amp or fifteen volts at one-third of an amp, both of which have a power dissipation of five watts. Since these specifications vary depending on manufacturer and the type of breadboard, you should check the data sheet before purchasing your breadboard. Due to the temporary nature of the contacts, most breadboard has a current limit of one amp or less."

https://electronics.stackexchange.com/questions/223702/what-is-the-current-carrying-capacity-of-a-single-stripboard-track

 

Offline tooki

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Re: Resistance of resistor leads ?
« Reply #8 on: June 23, 2019, 01:48:24 pm »
Did you paste the wrong link possibly? The page you linked to does not contain the quote, it's about perfboard, not solderless breadboards.

Moreover, that quoted text makes no sense... yes, 5Vx1A=5W and 15Vx0.3A≈5W, but the breadboard isn't dissipating that! Or rather... if your breadboard has 5 ohms of resistance in a contact strip, there's something massively wrong with it!!!! I tested a few of my breadboards, some eBay, some 3M/ASP, and other than one really bad eBay one, they all have around 9-10mOhm resistance.

Also, I've checked, and I can't find any breadboards with actual stated current/voltage specs. The data sheets do not specify.
« Last Edit: June 23, 2019, 01:52:02 pm by tooki »
 

Offline ArthurDent

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Re: Resistance of resistor leads ?
« Reply #9 on: June 23, 2019, 04:01:25 pm »
Quote
"Did you paste the wrong link possibly? The page you linked to does not contain the quote, it's about perfboard, not solderless breadboards."
The quote refered to solderless breadboards, like the one pictured above, and the link refered to perf board, like the one pictured in the link. Sorry if that wasn't clear. I understood it.

Quote
"I can't find any breadboards with actual stated current/voltage specs."
As to solderless breadboards having current ratings, check the link below where one well known supplier rates this board of theirs at 1.5A. The info is out there if you search for it.

https://globalspecialties.com/media/k2/attachments/EXP-350E_datasheet.pdf

The point of my previous post was that only paying attention to the wire size was ignoring a very important aspect of testing a circuit either on perf board or breadboard. Within the safe current rating of either type of board I avoid using wire that is too small and could cause contact problems in breadboards. Note that breadboards may also have frequency ratings that can cause other problems. I stand behind what I posted.

 

Offline tooki

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Re: Resistance of resistor leads ?
« Reply #10 on: June 24, 2019, 12:26:04 am »
Well, “the data” isn’t out there for most breadboard. I stated that I hadn’t seen any that specify it, and that was true. Believe me, for the 3M ones, I’ve looked. It’s not out there. (And I’m not even going to try looking for cheap Chinese ones, since their specs would be unreliable anyway.) A few others I looked at didn’t mention it, either. That one vendor does specify it doesn’t mean that all the rest do.

Ok, so what is the source of the breadboard quote with the nonsensical dissipation thing?

As for breadboards’ limitations: please, please let’s not start on this again. It’s not necessary for every thread where the word “breadboard” appears to devolve into a holy war about their merits and limitations. The OP wanted to know about wire ampacity and that’s been explained quite well.
 


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